Cathode-ray screen



Fgb. 3, 1948. 7 2,435,435

CATHODE RAY SCREEN Filed April 6, 1943 MODIFIED HALIDE SCREEN snom''PERSIVSTENCE on HIGH CONTRAST Invent of: Gov'bon RT-onda,

y 'His Attorney.

Patented Feb. 3, 1948 CATHODEFRAY SCREEN Gorton R: Fonda; SchenectadmN.-Y., assignor to GeneraL-iElecti-ic Company,

wii mk a corporation. of 1' Application April 6,1943, Serial No. 482,025lorfllaims; (Cl. 250:.-16l)i The 1 present invention relates -to cathode--ray devices of the type. having screens which-respond to, excitationbyicathoderays; ire; .abeamof electrons, .and; in. particular,- toscreens in which a marked darkening is producedbythe-impact ofelectrons.These devices are sometimes referred to as darktrace tubes-because thepath of the cathode ray beam is reproducedvin theform oi a.

dark trace, the intensity of which varies with alterations in theintensity of thec'athode ray beam. They are also-knownas-skiatrons. Thescreens of such devices commonly consist of an alkali metal halide, forexample potassium chloride.

Itxisthe object of-my present invention to improve alteration of thecomposition and characteristics of. the electron-sensitive screen Inskiatron tubes, suchasaused fo'rexample in rangeefinding :.and.-direction-finding equipment, a beam of. electrons or cathode rays movesover the screenin responseto an externallyap plieddeflecting'forcegrsuchas the-variation of an" electromagnetic field;Reception of electromagnetic. radiations which-are I reflected fronranobj act. to.- be located results in. appropriate current: variations inthe-control field of the equipment. As a result the cathode beam iscaused to travel over thescreeni-of the skiatron,--thereb3' producing adarki-imageor trace. This image or dark trace 'may'beviewed by lightwhich falls on. the. screen; fromtan. external source.- If desired, tthe image may be projected ona distant viewing surfaces. The4externaliillumination of the screennottonly permits -the-"dark -trace--to be viewed,.but. also has the-important function of causingthe .darktraceto decayand disappear,

thus restoring the original condition of the screen. In other'words;impinging light wipes out the dark-trace after it has served itspurpose.

and extend I the applications to- Whichskiatrons can beput; this resultbeingattained-by For some purposes it is desirable that thelife orpersistence .of xsuch-trace-should be reduced.-

Commonly,; the: dark: trace :remainsnvisible for about a minute, or:even1onger depending upon the intensity of the iilluminationj For somepurposes-itisxdesirable that the dark trace shoulddisappear .inaboutfive seconds. For still other purposes,: a decay period I of nolonger than one second is needed;

It'isv also desirable that thecontrast intensity between the .dark traceand the lighter background should be as marked as-possible to permit"ready recognition of the'trace or image? It is the primary object o--myinvention to viewed-in a darkened room,

reduce the decay periodof traces "or images on skiatron fili'ns- Such areduction not only widens the field' ofusefulness 'inrange-findinganddirection-finding equipment, but also renders it adaptable formse inother: fields.

tant field foninstance, would be television A- darktrace screenhavingasuificiently short--per-. sistence offers thenoteworthy-advantage over thescreen" that it need-' notbe J as-isnecessary for of screen, but may be observed present fluorescent thelatter type I equally well in a well-lighted room';

This shorter persistence is obtained iZbY aSSO ciating a screenconsisting of an. alkali halide with l a =minor amount J of a =-modifier-consisting of a example the halides of alkaline earth metaI including.magnesium, barium, cal

foreign halide, for

ciu'm and others which will be "mentioned here after.

I findthat the presence oithe foreign halide in an amount suflicient toreduce the decay-period I of "the dark trace alsoresults .in somesacrifice ofintensity of the dark trace. A decrease in the amount offoreign halide is accompanied by 4. a lessening of botheficcts; that.is,.the decrease in decay period and the decrease ofiintensity both:-

become less as the amount of modifier is reduced.- Surprisingly, these.variations .do not change continuously with lessening amounts of f-oreign additions.--v The presence of a very small amount of the foreignhalide varying from a few -hundredths of one per cent to a per cent ormore, depending upon the 1 activity oi the particular halide used, mayresult ina markedincreaseinintensity when the concentrationof theforeign halide has been reduced to such a point-thatitno longerafi'ectsthe lengt'hof the decay period.-

These and other: features of my invention will be described with greater-particularity hereinafter.- The accompanying drawing showsconventionallya cathode 1 ray" tubecontaining *a screen embodying myinvention.

The cathode ray tube here shown has as usual a relatively slendertubular portion 3 which communicates with'a bulbous portion 4; one wall5 "of which approximates a flattened shape. The dark tracescreenfifwhich responds to electron impact,

is-coated on the inner surface of the wall 5. At,

the opposite end of the tubular portion remote from the screen 6 is aso-calledelectron gun 1 for generating a desired cathode ray beam.

This electron gun, which of, course is sealed within the enclosingenvelope, may comprise an oxide-coated cathode 8 which is sh'own as a.small cylinder receiving heat "from an electric heater embodying myinvention as a control grid; Itiis connected to. an inter, mediate, morepositive point of potentialf the source l2 by a conductor l5. Anelectrode which may be a film (not shown) )on theinterior of f theenvelope wall is connected by a conductor. [6.

The device of the drawing is" i shown only as a schematicrepresentation'il'lus-' to the source l2.

trating generally the type of device in'which films are used and,therefore, beam-,defiecting means and vices have not been shown.

The composition comprising halide and 'modi-i fier, the preparaion ofwhich will be described, may be applied as a film 6 on the insidesurface of the end wall of the skiatron shown in the draw-,

ing bysublimation, from a holder l9, indicated schematically as beingheld by a spring wire support 20. The modifier mixture, after fusion ashereafter described, may conveniently be ground and compressed intoarpellet for insertion in the holder the exhaust of the device, heat maybe applied by,high frequency induction to effect the'sublimation..

In the preparation of the material to be volatillzed and applied as askiatron or dark trace film an alkali halide is mixed inwthe finelydivided state. with a desired amount of chosen finely The mixture issubjected to; fusion which is sufiiciently prolonged to bring dividedmodifier.

about complete homogeneity. I A fusion, temperature should be maintaineduntil the melted mass has become clear, transparent, and free from theagitation which occurs when a fusion heat is first applied. However, thefusion should not-be prolonged so long as to cause undesired .chemicalchanges, such as oxidation. Also it is undesirable to havethetemperature rise much above the melting point .ofthe mixture. Potassiumchloride, potassium bromide, rubidiumchloride, lithium iodide and sodiumchloride maybe employed satisfactorily as the main constituent of thescreen. ,I prefer touse potassium chloride be cause it yields thehighest intensiy of dark trace. 1 The modifying material, particularlywhen. employed with potassium chloride, advantageously may consist. ofvarious, halides such, for example, as the chlorides of Samarium,thallium, manganese, sodium, magnesium, cadmium, lead, chromium, nickel,platinum, caesium, calcium, aluminum, iron thorium, lithium, rubidium,copper,

barium, zinc, zirconium or tin.. The extent of the 6O and alsowith thehalogen chosen. Frequently activity varies with the metallic elementchosen the iodide is more active than the bromide, and the latter moreactive than the chloride, but this is notinvariably so. in activitywhich increase with concentration over a certain but variable range, itis generally pos-' sible to obtain the same shortening ofpersistence byadding a small amount of anactive halide of a foreign metal as by addinga. relatively large amount of a less active one. Ordinarily, theaddition of such modifier will be less than ten per cent by weight. A

Such mixtures of only because of the extensive reductions inperother-auxiliary doy forme d by a beam of l9. At an appropriate pointduring Because of these variations 7 there is no longer any'particularon the persistence-but the intensityis increased f V V H,that;;'obtainable with potassium chloridewith mag-V nesium chloride areespecially-advantageous not sistence that are obtainable'but alsobecause, these reductions are accompanied by smaller losses in theintensity of the dark trace. A further advantage of films prepared fromthese mixtures with 5 magnesium chloride lies in the speed with whichthe "after trace disappears; that is, a long persistent trace of low-intensity characteristic of films of potassium chloride{alonejajfsfwell' as of films prepared from many of its mixtureswith 10foreign halides.

- Because of the variations in activity character- *'istic of thedifierent modifiers, the proportion of modifier should be chosen to givethe resultsdesired, theperiodofdecay (or persistence) becomingprogressively shorter (within limits) with increase of modifier. "Forexample, the dark trace V electrons on a film of potassium chloride,unmixed with a modifienpersists,

F under'a certain intensityof illumination, at least about two minutes.The following tableshows the effect on persistence, under the sameintensity of illumination, of films vaporized -from fused mixtures ofpotassium chloride withprogressively largeradditions'of hydratedmagnesium chloride:

' Length in Calculated as Per Cent MgCla by Weight I As above stated,the additionof a small amount 5ml platinumhalide to potassium chlorideresults in a composition which, when volatilized, forms a-film ofshorter persistence thanunmodified potassiumchloride.

Shorter persistence also. is exhibited by 1 ;,made by volatilizing asdescribed a fusion ofa mixture of potassium chloride and "potassiumhydroxide in a platinum containerJ The fusion is enveloped either in ahydrogen atmosphere or in air during preparation. The fusion-may resultin some complex volatile compound ofpotassium chloride, potassium 1hydroxide: and platinum. Such compoundappears to 'be the active agentin. volatilization'of suchfusion a film prepared bythe as abovedescribed. a j As already observed; the intensity of: the initialcentrations of these foreign halides ,much below these required foriobtaining' any notable shortening of persistence. For mixtures ofpotassium chloride .with magnesium chloride, for instance,

to 0.02 per cent magnesium chlorideresults, in only*a:normal persistadecrease in concentration once but the intensity of trace fincreases 25:per cent.

, by about 4 0,per cent over potassium chloride alone,- .7

Seconds of 1 Decay Period dark trace may beincreasedby reducing the con-A still greater'intensity or contrast'is obtained 1 potassium chloridewith barium effeot observable I a What I claim as new and desire-Rtesecure by ,7 5- Letters 'Patent of the United States. is; :j

saunas 1. A cathode ray device containing a screen consisting of a blendof alkali halide and a halide or alkaline earth metal, the latter halideconstituting less than ten per cent of the entire composition.

2. A cathode ray screen consisting mainly of halide of an alkali metaland an associated chloride of the alkali earth metal constituting nomore than about five per cent by weight of said alkali halide.

3. A cathode ray screen consisting mainly of potassium chloride and nomore than about five percent of magnesium chloride.

4. A cathode ray screen consisting mainly of potassium chloride and nomore than about five per cent of barium chloride.

5. The method of preparing a cathode ray screen which consists insubjecting to fusion 9. mixture consisting mainly of halide of an alkalimetal and up to about ten per cent or an alkaline earth halide,maintaining the temperature adjacent the melting point until agitationceases and a clear, quiescent fusion results, and subliming theresulting product on a receiving surface in a cathode ray tube.

6. The method of preparing a cathode my screen which consists insubjecting to fusion a mixture consisting mainly of potassium chlorideand up to about five per cent of magnesium chloride, maintaining thetemperature adjacent the melting point until agitation ceases and aclear, quiescent fusion results, and subliming the resulting product ona receiving surface in a cathode ray tube.

7. A cathode my device comprising a glass envelope. 9. source of cathoderays which are oriented to impinge on a portion of the inner surface ofsaid envelope and a coating on said surface consisting of a pre-fusedmixture of alkali halide and alkaline earth metal halide, the latterconstituting about 0.3 to 5 per cent by weight of said coating.

8. A cathode ray screen consisting mainly of a mixture of potassiumchloride and magnesium chloride, the latter constituting about .3 to 1.0per cent by weight of said mixture.

9. A cathode ray screen consisting mainly of potassium chloride andapproximately five percent of magnesium chloride.

. 10. A cathode ray screen consisting mainly of potassium chloride andapproximately 5% of a chloride of a member selected from the groupconsisting of magnesium, zinc, and barium.

\ GORTON R. FONDA.

REFERENCES CITED The following references are of record in the

